The present invention relates broadly to extrusion granulators and more particularly to a screw-type extrusion granulating apparatus with a spherical die in axial alignment with the screw, which is especially suitable for efficiently producing granules of very small cross-sectional dimension.
Extrusion granulators are widely employed in the pharmaceutical, food, agricultural, chemical, and ceramic industries where moistened powdered raw materials such as chemicals, fertilizer, or feed stuffs to be formed into granules. Basic screw-type extrusion granulators utilize some form of rotating screw to transport the moistened powdered material to a die having holes formed therein. The pressure and scraping action of the screw forces the moistened powdered material through the holes in the die through which the material emerges in granulated form. "Front" type extrusion granulators have the die located in axial alignment with the conveying screw.
Conventional extrusion granulators of this type consist primarily of four main elements including a single or twin conveying screw arrangement driven by a motor, a screw housing in which the conveying screw is disposed, an extrusion blade or blades mounted to the front end of the conveying screw, and a flat die plate attached to the front end of the screw housing in axial alignment with the screw. In operation, powdered raw material previously moistened and plasticized by kneading machines is inserted into the screw housing from a feed hopper. The raw material is then forced to transfer forwardly to the front end of the screw housing by means of the conveying screw. During this process, the raw material is pressurized and compressed by the conveying screw and is extruded continuously through multiple die holes in the die plate.
Internal extruding pressure for extruding the raw material through the flat die plate has its maximum pressure in a uniform pressure region located just prior to the die plate. Therefore, the die thickness is selected to have sufficient strength to withstand the maximum extruding pressure. Typically, a smaller sized conventional extruder having a screw diameter in the range of 60 millimeters to 130 millimeters will have a die plate of a thickness of at least 5 millimeters, while larger extruders with screw diameters in the range of 180 millimeters to 300 millimeters have a die plate of at least 8 millimeters in thickness.
Extrusion granulators with flat die plates of this order of thickness achieve satisfactory production capacities when their die holes are relatively large so as to extrude correspondingly large granules. However, at a constant die thickness, as the die hole size is reduced to produce smaller sized granules, the production capacity of such extrusion granulators is reduced to unacceptably low levels and ultimately causes clogging of the die holes due to increased frictional drag imposed on the extrusion flow of material through the die holes. Generally, it has been found that a conventional flat die extruder having a die thickness of 5 to 8 millimeters can extrude raw material through a die hole diameter below 1.5 millimeters only with a very small production capacity which is normally far below acceptable industrial production rates.
This problem is becoming increasingly acute as smaller granule sizes are becoming increasingly required in various industries. For example, the pharmaceutical, agricultural-chemical, and catalyst industries are increasily demanding the ability to produce granules of less than 1.2 millimeters in diameter, often less than than 1.0 millimeter diameter, and even as small as 0.3 to 0.6 millimeter diameters. In the agricultural chemical industry, smaller herbicide granules, say in the range of about 0.7 to 1.2 millimeters can be distributed more uniformly than larger sized granules, enabling a lesser quantity of the smaller granules to achieve the same herbicidal effect as a greater quantity of larger granules, thereby reducing herbicide costs and minimizing any attendant environmental hazard. Likewise, in the pharmaceutical field, smaller sized granules improve drug availability and dispensability. In the catalyst industry, smaller granules provide increased surface area per unit weight, enabling equipment to be downsized and economy and efficiency of operation to be improved.
It has been proposed to reduce the die thickness to correspondingly reduce frictional drag and, in turn, achieve a higher production capacity. However, in tests employing a punched die plate having a punched die hole diameter of 1 millimeter with a die plate thickness of 1 millimeter and an opening ratio of 22.4%, although the production capacity of the granulator was significantly increased temporarily, the punched plate deformed outwardly after several minutes of operation, which led to a wider clearance between the punched plate and the extrusion blade, resulting in a lower production capacity and finally destruction of the punched plate. The primary cause of this result is that the relatively thin punched plate is not sufficiently strong to withstand the internal pressure generated within the screw housing.
In an effort to solve this problem, another type of extrusion granulator known as a screw-type radial extrusion granulator has been developed for production of smaller diameter extrudates in the range of 0.7 to 1.2 millimeters. In this type of granulator, an extracting screw is mounted to the conveying screw in a screw housing having a die holder which supports a generally semi-cylindrical punched plate laterally outwardly about the extracting screw. In this type of extrusion granulator, the raw material is first conveyed and compressed by the conveying screw axially within the screw housing and then is forced radially outwardly through the semi-cylindrical die by means of the extracting screw. A problem with this arrangement is that internal pressure generated by the conveying screw within the housing cannot be fully employed for the extrusion of the raw material through the die because the raw material is forced to change direction from a generally axial flow to a movement directed radially outwardly from the extracting screw, thus resulting in energy loss and inefficient production of granulate. Accordingly, the production capacity of this radial type extrusion granulator is far smaller than that of the front-type extrusion granulator.